The present invention relates to glass-ceramic joining materials which can be processed at low temperatures and are suitable for sealing, especially of fuel cells or electrolysis cells at low joining temperatures, and further applications.
Glass-ceramic joining materials are understood in the context of the invention to mean materials which originate from an amorphous glass material, crystallize at least partly and are then in the form of glass-ceramic. These glass-ceramic joining materials can be used as glass solders, but also in the form of preforms etc.
Glass-ceramic joining materials are used especially for production of bonds, in order to bond particularly ceramic components to one another or to bond them to metal components in an electrically insulating manner. In the development of glass-ceramic joining materials, the composition thereof is often selected such that the coefficient of thermal expansion of the joining materials corresponds approximately to that of the components to be bonded to one another, in order to obtain a permanently stable bond. Compared to other joining materials, for example those of plastic, glass-ceramic joining materials have the advantage that they can be configured so as to give a hermetic seal, can withstand higher temperatures and degenerate less, if at all, even in the course of longer service lives.
Joining materials in the form of glass solders are generally often produced from a glass powder which, in the joining operation, also called soldering operation, is melted and gives rise to the bond with the components to be bonded under the action of heat. The joining temperature is generally selected at about the level of what is called the sphere temperature of the glass. The measurement of the sphere temperature is a standard test method known to those skilled in the art and can be effected with a heating microscope. If a crystallization-free glass as a joining material in the form of a glass powder is melted and cooled again such that it solidifies, it can typically also be melted again at the same melting temperature. For a bond with a joining material in the amorphous state, this means that the operating temperature to which the bond can be exposed for prolonged periods must not be higher than the joining temperature. In fact, the operation temperature in many applications must be significantly below the joining temperature, since the viscosity of the joining material decreases with rising temperature and a glass which is free-flowing to some degree can be pressed out of the bond at high temperatures and/or pressures, and so the bond can fail.
This problem can be solved by using glass-ceramic joining materials in which the amorphous base glass crystallizes at least partly or else completely in the course of the joining operation. In the context of the invention, partly or fully crystallized joining materials are called glass-ceramic joining materials or glass-ceramic for short. The crystalline phases or the ceramics generally have properties differing significantly from the amorphous base glass, for example with regard to thermal expansion or glass transition temperatures, such that the overall system composed of amorphous glass phase and crystalline phases, i.e. the glass-ceramic joining material overall, may likewise have different properties from the amorphous base glass alone. More particularly, the temperature required for the re-melting in the case of glass-ceramic joining materials may be well above that of the amorphous base glass.
Whether an amorphous base glass gives rise to an amorphous glass or a glass-ceramic in the joining operation, given suitable composition of the base glass, depends to a high degree on the process conditions in the bonding operation, more particularly on the heating and cooling curves.
One field of use of joining materials is, for example, that of bonds in fuel cells which can be used, for example, as a power source in motor vehicles or for localized energy supply. An important type of fuel cells is, for example, what are called solid oxide fuel cells (SOFCs). The bond with the joining material is typically used to produce fuel cell stacks, i.e. for the bonding of several individual fuel cells to give a stack. Such fuel cells are already known and are continuously being improved. High temperature fuel cells are widespread, these comprising particularly expensive materials which are difficult to produce, more particularly specific metals and/or alloys. Therefore, in the current development of fuel cells, lower operating temperatures are desired, especially because it is thus possible to use comparatively cheaper ferritic stainless steels which are easier to produce (for example AISI 410, AISI 430), and the use of specialty alloys such as Crofer22APU (Thyssen Krupp) or ZMG32 (Hitachi), which is otherwise necessary, is thus no longer required. At the operating temperatures which then occur, however, it is also necessary to use different cell materials and electrolytes since oxygen ion conductivity is otherwise not ensured. The materials used for low operating temperatures in the fuel cells require sealing materials that can be processed at low joining temperatures.
Apart from the known glass solder sealing, there are other sealing materials, but these entail considerable disadvantages. Mica seals might lead, for example owing to the alkali metal content, to increased oxidation of the metal, and provide sealing only under high pressures. Hermetic bonds are not achievable in this way.
The barium silicate glasses known to date have too high a softening temperature and joining temperature in order thus to be able to produce bonds at low joining temperature.
U.S. Pat. No. 6,532,769 describes joining glasses having a SiO2 and B2O3 content of at least 35 mol %. Such a high proportion of SiO2 lowers the coefficient of thermal expansion of the joining glass and leads to an unwanted increase in the melting temperatures.
WO 0294727 A1 likewise discloses SOFC joining glasses. Here, the glass compositions have SiO2 contents between 33 and 35% by weight. The additions of PbO, MnO, V2O5 are undesirable because they are very substantially redox-stable. Especially elemental lead which forms is capable of forming alloys with the metallic interconnector and thus has a corrosion-promoting effect. MnO and V2O5 are likewise polyvalent elements. Mn and Cr from the ferritic steel can likewise enter into a multitude of bonds, the stability of which under operating conditions is questionable. The glasses mentioned are designed for joining temperatures above 800° C.; they cannot be used to achieve joining temperatures of 800° C. or less.
U.S. Pat. No. 6,124,224 discloses barium-containing glasses whose SiO2 contents vary between 18 and 60% by weight. The joining temperatures of the glasses described are more than 900° C., and the coefficient of thermal expansion is between 5.0 and 7.4 ppm/K. Because one or both of the SiO2 and Al2O3 contents are very high, they are unsuitable for fuel cell applications; more particularly, the thermal expansion values are not matched to the metals and cell components.
The glasses disclosed in DE 19857057 C1 have a high MgO content, and the SiO2 content is between 35 and 55 mol %. Glasses having a high MgO content have very significant crystallization, and the high SiO2 contents do not allow joining at temperatures of less than 800° C.
SU 802220 A1 describes barium-containing glasses suitable for soldering of titanium. In this document too, the SiO2 contents are more than 32% by weight, and the addition oxides TiO2 and TeO2 are used. TeO2 is an adhesion promoter and is said to increase chemical stability. The joining temperature specified is 1100° C.
EP 2135316 B1 describes barium-containing semicrystalline sealing glasses in solid oxide fuel cells containing at least 10 mol % of SiO2.
None of these glasses are suitable for the production of bonds at a joining temperature not exceeding 800° C.